Decarbonization of spent acids



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Oct. 11, 1960 A. BELcHE'rz DECARBONIZATION OF SFENT ACIDS Filed Oct. 14,1955 ARNOLD BELcHETz INVENTOR swg@ tains less than 1% carbonaceousmatter.

United States Patent DECARBONIZATIGN 0F SPENT A'CIDS Arnold Belchetz,Larchmont, N.Y., assignor to National Distillers and ChemicalCorporation, New York, N.Y., a corporation of Virginia Filed Oct. 14,1955, Ser. No. 540,495

8 Claims. (Cl. 23-172) The present invention relates to a process forregenerating spent acids and, more particularly, to a process `fordeodorizing and decarbonizing spent sulfuric acids containingcarbonaceous matter.

In many processes utilizing sulfuric acid, spent sulfuric acid isobtained contaminated with carbonaceous matter that imparts undesirablecolor and odor characteristics to the acid. This contamination rendersthe spent acid unsuitable for further use in the process from which itwas obtained and for other uses, unless the carbonaceous matter issubstantially or completely removed. Illustra- Vtive of processes fromwhich such carbon containing sulfuric acids are obtained are alkylationprocesses, in which sulfuric acid is used as a catalyst for alkylationof isoparains with olens; petroleum refining processes, in whichsulfuric acid is used for treatment of petroleum fractions, such ascracked distillates and lubricating oils; and nitration processes, inwhich sulfuric acid-nitric acid 4mixtures are used for the nitration ofaromatic hydrocarbons.

Spent sulfuric acid is also obtained from olefin hydration processes forproduction of alcohols, in which an olefin, such as ethylene, isabsorbed in sulfuric acid to form the sulfuric acid ester of theoleline. The acid solution of the ester is diluted with Water andhydrolyzed to form the alcohol and sulfuric acid from the ester. Thealcohol is removed from the dilute sulfuric acid solution l=bystripping, usually with the aid of steam. The residual colored,relatively weak acid containing about 50% H2804. The spent, strippedalcohol acid usually con- If the spent -acid is allowed to settle, alayer of sludge acid, containing a relatively high proportion ofcarbonaceous matter, separates on top of the main bulk of the spent 50%acid. This alcohol sludge acid contains about 5 to 7 weight percent ofcarbonaceous matter in 50% H2804.

The bottom layer of spent 50% acid is generally concentrated to an acidstrength of about 78%. This partially concentrated acid, commonlyreferred to as alcohol black acid, generally contains less than onepercent by weight of carbonaceous matter and is potentially useful forfertilizer manufacture, in processes such as the acidulation ofphosphate'rock to produce superphosphate. It can also be furtherconcentrated or fortified with sulfur trioxide to higher strength, forreuse in the olefin hydration process from which it was derived.Although such alcohol black acids are potentially useful for thesepurposes, their utility therefor is impaired by the presence of thecarbonaceous matter, which causes undesirable color and odorcharacteristics, even thou-gh the carbonaceous matter content may be aslow as about 0.3 weight percent. The alcohol sludge acids containing 5to 7% carbonaceous matter, are markedly impaired by their high contentof carbonaceous matter and are often disposed of as waste, presenting awaste disposal problem, in addition to a loss of valuable acid values.So far as `I am aware, an effective, economically attractive method .y2,955,920 Patented Oct. -1,1, 1960 ice for reconditioning alcohol sludgeacids, as substantially decolorized, decarbonized sulfuric acid, is notavailable.

The main purpose of the present invention is to provide an eicient,economical process for decolorizing and decarbonizing spent sulfuricacid containing carbonaceous matter and to produce therefrom regeneratedacid of enhanced value and utility. Other objects and ad-` vantages thatresult from practice of the present invention will be -apparent from thedescription of the invention which follows.

In accordance with this invention, spent -sulfuric acid, containingVcarbonaceous matter, is effectively decolorized and decarbonized bycontacting the spent acid, in which the proper amount of a vsuitableoxide of nitrogen is dissolved, with a mixture of an oxygen-containinggas and sulfur dioxide, under conditions whereby the carbonaceous matteris effectively oxidized and removed from the acid and the sulfur dioxideis oxidized to sulfur trioxide. More specifically, the process comprisescontacting an aqueous solution of spent sulfuric acid, containing insolution approximately 3 to 10 weight percent `of a suitable oxide ofnitrogen, with an oxygen-containing gas, such as oxygen or air, andsulfur dioxide, under conditions whereby the sulfur dioxide and aportion of the oxygen dissolve in the acid. Part of the dissolved oxy--gen is used to oxidize the dissolved S02 .to S03 in the liquid phaseand the balance of the dissolved oxygen serves to oxidize thecarbonaceous matter to CO2 and water. The S02, which is formed, reactswith water which is present in spent acid feed or which is formed byoxidation of carbonaceous matter, to form sulfuric acid. Following thedecarbonization treatment, the sulfurie -acid is stripped of dissolvednitrogen oxides, to obtain a decarbonized, decolorized productcontaining the sulfuric acid originally present in the spent acid fed tothe system, as well as the additional sulfuric acid produced from S02during the decarbonizing treatment.

Still more specifically, the invention relates to a con'- tinuousprocess in which spent sulfuric acid, containing carbonaceous matter, iscontinuously added to a solution of dinitrogen trioxide in sulfuric acidof from to H2804 content, and the resulting acid mixture is contactedwith an oxygen-containing gas and sulfur dioxide, under conditionswhereby oxygen and sulfur dioxide `dissolve in the -acid mixture,carbonaceous matter is `oxidized to carbon dioxide and water, sulfurdioxide is oxidized to S03, and S03 reacts with water present in thespent `acid feed or formed in the oxidation of the carbonaceous matterto yform sulfuric acid, thusmaintaining the sulfuric acid content of theacid mixture at a strength of from 80 to 90% H2504 In preferredembodiment, decarbonization of spent sulfuric acid is effected underconditions where the acid being decarbonized is maintained Vat astrength of about 82 to 86% H2804, with approximately 5 to 7% by weightof `dinitrogen trioxide in solution in the acid, by contacting the 82 to86% strength acid containing N203 and carbonaceous matter, with air andsulfur dioxide at a temperature of approximately to 300 F., or higher.Sufficient air is used to oxidize the sulfur dioxide to sulfur trioxideand the carbonaceous matter to carbon-dioxide and water and, preferably,to provide an excess of about 5 mol percent of unreacted oxygen in theresidual gas from the decarbonization operation. Water in the spent acidfeed or `formed by oxidation of the carbonaceous matter in the process,is utilized for reaction with the sulfur trioxide formed by oxidation ofS02 in the system. Suicient S02 is oxidized to S03, to maintain the acidstrength at from 82 to 86% H2804 for the decarbonization operation.

Further embodiments of the invention relate to the use of the feedgases, obtained by burning sulfur with an eX- bnized acid, and to theuse of the gases from the deiiitration operation for supply of sulfurdioxide, nitrogen oxides and :oxygen .to the decarbonizing operation.

' :To .illustrate practice. of .an embodiment V.ofthe invenieferenceishmavde to the accompanyingv drawing, Whichis a `s'cheluaticillustrationV of an yassemblage of ape paratns. v'suitable for ypractice.of the invention.. 'With reference to the aspect ofthe inventionrelating todecarbonization of 'spent sulfuric acid containingcarboriaceous matter, `decarhonization may be carried out in tower .10which, fas shown, containsthree Vpacked sections. The bottom section of'towerY 10 serves asareactor-oxidizer, in'which the Ycarbonaceousmatterin the spentn acid feed is oiidizedandfin which a's'ubstantial amountYof .the-S02'. fintnodc'ed intoY the .tower converted to sulfuricY acid.

middle sectionservesas a .reactor to4 convert .the remainder of theS0210 Vsulfuric acid. The vtop section of tower I0 serves .as .anabsorberfor the nitrogen oxides 'which are contained -in the gasesleaving .the middle section.` These nitrogen oxides .are absorbed almost.cornpletely in the spentsulfuric acid fed to tower 10, as describedhererina'fter. l Spent acid feed s introduced into tower abovethev topsection through line 11. yAV gaseous stream, containing oxygen, yoxidesof nitrogen and S02, is introducedint tower 10 Abelow the bottom sectionthrough .line 22. De-

acid is withdrawn from tower r10 through line 17, located at the bottomof the tower, .and residual :gases are withdrawn from tower 10 vialinelZ, located atthe top" of the tower. The .decarbonized acid,withdrawn from tower 10 through line 17, is passed through cooler C-2into line 18. VPart of the acid from line 18 is recycled .to .tower 10through line 15 and part is withdrawn from the decarbonization operationthrough line 19. The acid recycled to .tower 10 through line 1S, isfurther divided hetweenlines 14 and 13. The acid passing through line14, .enters .towerr10 midway between the middle and bottom sections. TheVacid from line 13 passes through the cooler .G-l and enters tower 10between the top ,and middle sections'.

, The following example illustrates practice of the in- )ention forcontinuous decarbonization of 78% strength alcohol .black acid, havingthe following composition, and introduced into tower. 10 at atemperature at'9'0 F. through .line 11, at the rate of approximatelylOOtons kH2S04 perday (100% basis).

lohol lla'clk ac'd feed entering tower 10 through [irre Il oinposiuonWaper- Lbs/hr. "ro'nsf cent day 10 at a rate o'f 10,352 pounds per houror approximately AZl00 c`:.f.rn'. measured at the owing conditions of37009 F.

and 6 "p.s.i.g.' pressure. The'gas owsupwardsthrough e tower, thuscontactingthe downward owing acid,

introduced throng@ unes 11,712 and 14.`

Composition and quantity of gas introduced in tower 10 hrough line-22v YMol per- Mols/ Lbs/hr.

cent hr.

3.3 1o. 3.3 1o. 9.1. .29. Y l19.4 a3. j...7.379 .todo 'i The gas fromline 22, entersv tower 10 at approximately 300 F. and is cooled to`about T60 F. as it'passes upwardly through the bottomsectionof ihntower. In the Ybottom section, the gas is subjected to intimate contactwith a mixture of acid, Apart of which is acid flowing from the middlesection to the bottom section at a temperature proximately 160 Rand partof which is 'acid eir- 'of a cnla'ted to tower 10 through line 1-4 tl60"F." ltleil cools the gas to approximately `160" P., before passes fromthe bottom Yto the middlefse'ction, and; lin doingfso, the acid absorbspartoff the-N0, NO2-,S02 and oxygen which enter'tower lo'through'lin'e2-2; `N0

`and N02 are absorbed in approximately equilnolar tities inthesulfuic-acidY and-combine insolution lto dfox-rn N203, which in lturn"forms estable complex with 150g or 'H2-S04. yThis complex has a verylow Vapor pressure in sulfuric acid of about y85% s'trengthand is-alver'y power'- ful oxidizingagent. While I do not wish yto'be anytheories relating to Ithe nature of this ycoxr'lpl'ez'i,'dit is believedthat thecomplexhas 4a composition ing to the formula`N203.2S03. Y

Approximately half of the lSO2 which enter'stower 10 through line Z2is'abso'rbed'in the acid in theibottrn'sec- Vtion and isimmediatelyoxidizedto S0 3 4Vby the-Nithin the acid solution.v The N203in'turn 4is lreduce'dto ZNQ, which is then inunediately reoxidi'zed toN203 by oxygen which dissolves in the 'acid solution. Thus, theneteectlis the oxidation lof S02 'to'S'Qa by oxygen Cwhich isy'absorbed withthe S02`in sulfuric acid-N203 solution'.`

. The S03 formed combines with water in the acid .tion to form H2804. An f f The N203 vin the .acid solution also loxidizes thecarbonaceousmatter .in the V'acid passing from the middle V:section of tower l10 tothe bottom section. The cafbonaceous .matteris oxidized to {C0-2andi-1,20 .by the N203, which is reduced to .2N0 andis :again reoxidized.to N203 by oxygendissolved in the acidtroin the gas. Thus, the net`elect .is .the v.oxidation of car bonaceous matter zby means vof oxygencontained ,inV .the

gas entering tower 10 through line'ZZ.

A number"V of highly exothermic reactlons .occur `in .the

nbottom Vsectionof tower 10, as .followsz (l) AThe reaction of N0 andN02 .to form heat of solution of N203 in sulfuric 'acid and theheatof`formation of the complex 15150325.03.

(2) The oxidationof S02 to S03 and the reaction of .S03 with water to`forni H2S04.

(3.) The heat Y*of dilution of H2804 formed to approximately H2804strength. 4 4 ,e .l Y

r,(4) Theoxidationof -carhonaceous matter in the acid to CO2 and H20.

All of -the Yheat evolvedin these 'reactions is absorbed by the acidlwhich contacts the gas. The acid Yalsoabsorhs the sensible heatliberated in cooling the gas from'300 to F. As a result thetemperatureof the acid .rises .as it flows down the bottom section,Afrom approximatelyI .160 lF. -to about'22'5 F. The acid withdrawnfrom-:the

bottom of tower'lt)Y .has a strength of approximately .85% HzSOrandcontains about 6 -wfperdent N203 in 18.91.1150@ ,oxidation ofcarbonaceous 84.5% H2S04. Ain cooling the gas entering the middlesection from 160 l matter proceeds very rapidly at a temperature of 225F.

Approximately 112,300 pounds per hour of decarbonized 85% H2S04containing 6% N203, is withdrawn from tower through line 17 and iscooled in C-2 to about 160 F. before entering line 18. About 88% of theacid entering line 18, or approximately 98,400 pounds per hour, isreturned to tower 10 through line 15, and the rest of-the acid,amounting to 13,880 pounds per'hour, is Withdrawn through line 19 fordenitration and recovery of the contained N203 for further use. Thedecarbonized acid withdrawn through line 19 has the followingcomposition: Y

Composition of acid entering line 19 0f the 98,400 pounds per hour ofacid entering line 15 at Vapproximately 160 F., 37,100 pounds per houris recycled to tower 10 through line 14 and 61,300 pounds per hourAenters line 13. The acid entering line 13 passes through the cooler C1where the temperature is reduced from 160 F. to approximately 122 F.From C-1 the acid passes through line 12 and enters the tower 10 at atemperature of approximately 122 F. at a point between the top andmiddle sections. The acid from line 12 is mixed with the acid ofapproximately 78% strength which flows from the top section to themiddle section at a temperature of about 122 F. The acid from the topsection consists of the alcohol black acid feed entering tower 10through line 11 and a small amount of nitrogen oxides, approximately 100lbs/hr., which escape from the middle section in the gas entering thetop section and are absorbed in the black acid feed in the top absorbersection. The following is the approximate quantity and composi- Ation ofthe acid at the top of the middle section.

F. to 122 F. In addition about 200 lbs./hr. of nitrogen oxides or about25% of the nitrogen oxides which enter Vtower 10 through line 22 areabsorbed in the acid in the 'middle section, with consequent liberationof heat in the formation of N203 and the reaction of the N203 with S03to form the complex N2O3.2S03. Against these exothermic reactions, someheat is absorbed in raising the H2804 content of the alcohol black acidfeed from 78% to about 84.5% H2804.

The H2504 produced from oxidation of of the S02, or 914 lbs/hr. SO2, toS03 will be approximately 1400 lbs/hr. in the middle section. This willremove 257 lbs/hr. of free water, by reaction with S03 to form H2802,Only a small amount of oxidation of carbonaceous matter occurs in themiddle section, as compared with the bottom section, due to the lowertemperature in the middle section. The composition of the acid leavingthe middle section will be approximately as follows, neglecting thesmall amount of water formed by oxidation of carbonaceous matter in themiddle section.

' Acid Entering Ab- Acid Leaving the Middle Section sorbed MiddleSection 1n Middle Wt. per- LbsJhr. Section, Wt. per- LbsJhr.

cent lbs/hr. cent (CH2) n 54 0.08 54 0 3, 625 200 5. 5 3, 825

In the bottom section the following reactions occur:

(1) About 50% of the S02, or 914 lbs/hr. S02, are oxidized to S03 andconverted to 1400 lbs. H2504 per hour, thus removing 257 pounds of freeWater.

(2) About 54 pounds per hour of carbonaceous matter is oxidized, forming170 pounds CO2 and 70 pounds water per hour.

v (3) About 500 pounds N203 per hour are absorbed in the acid.

Alcohol Black Acid Recycle Acid to Total Acid to from Top Section MiddleSection Middle Section Wt. Lbs/Hr. Wt. Lbs/Hr. Wt. Lbs/Hr. percentpercent percent (0H: 0. 5 54 0.08 54 N103?. 0. 9 100 c, 1 3, 525 5. 0 3,625

Total. 10, 820 61, 300 72. 120

It will be noted that the acid entering the middle section consists of84% strength sulfuric acid With about 5% N203 and 0.08% carbonaceousmatter in solution. As this acid ows down the middle section, thebalance of the SO2 entering tower 10 through line 22 and not absorbedand oxidized in the bottom section, is vabsorbed in the acid andoxidized to S03. By the time the acid leaves the bottom of the middlesection, the strength of the acid will have increased to about 84.5%H2804, the N203 content will have increased to about 6 wt. percent, andthe temperature will have increased to approximately F. The temperatureincrease is due to the absorption of heat liberated in the middlesection in oxidizing about half of the S02 entering tower 10 throughline 22 to S03,

Vthe reaction of the S03 formed with water to form H2S04 and thedilution of the H2504 formed from 100% to The acid also absorbs the heatliberated Y The composition of the acid leaving the bottom of tower 10will thus be as follows. The recirculation of l 7 A amount of nitrogen-oxides is lost Yfrom the .SYSm in the vent gas which leaves tower 1.0through line 1 2. YTo Ecompensate for this loss, an aqueous solutionVnitric acidY YVis injected into tower through line 16.

the `present case, l'the `loss of nitrogen oxides is yapproximatelylbs/hr. and-the following quantity of Solution of ,nitric acid is addedthrough line 16. The Initric acid decomposcsfto form N205, which in turnis reduced to N202.

Wt. per- Lbs/hr.

Cent

HNO. Y 43 42 H20, Y Y Y 51 55 .Tnfi 100 91 Component Molper- Mols/hr.Lbs/hr.

cent

Total 100.0 Y255.2 7,280

The gases which enter the top absorber section at 12.2 F., contain about125 lbs/hr. nitrogen oxides, as well -as the C02 formed in the`oxidation of the carbonaceous matter. The gases are cooled toapproximately 90 F. in the top section, while being scrubbed :by vthenon). I`the acid solution. Effective denitration cannot be obtained at suric acid strengthsA m-uch above 70% H2804, .owing to the stability ofthe chemical complex formed by N203 with S03 or H2S04. The addition o fwatertodilute Vthe acid to about 68 to 70% nso., .weakens .me mennenbond V.between No3 and S03 and Y'makes it possible to achieve almostVcomplete stripping of the nitrogen oxides from the sulfuric acidsolution. The denitration tower 38 contains two packed as indicated. Gasfor stripping the nitrogen oxides from the diluted acid enters tower 38through lineV 27 at 'a Ypoint located below the lower packed section.I'he gas for this purposeV is obtained Aby the combustion of with .anexcess of air .to produce SO2 in .the combustion 'chamber 36'.V The hotgases from combustion Vchunbe' 36 pass 'through line 29 into the acidconcentrator 39, where theheat .the easesis used toevaporate'water fromthe 68% strength denitrated sulfuric acid, in order to raise itsconcentration .to..Z8% H2SO4 lTheconcen- .tration of the sulfuric acidis effected under a pressure alcohol black acid feed. The acid in turnis heated to .v

-approximately 12.2 F. in cooling the .gases `and* in absorbing nitrogenoxides, before flowing from the top to the middle section. The acidpicks up labout 100 lbs/hr. of nitrogen oxides in the absorber section.represents approximately of the total amount of nitrogen oxides in thegas entering the top absorber section.

. The loss of nitrogen oxides amounts to approximately 25 lbs/hr., whichis approximately 3% ofthe total nitro- -gen oxides entering tower 10through line 22. The acid owing from the top of the middle sectioncontains approximately 0.9 -wt. percent N 203, as previously indicated.

The strength decarbonized acid withdrawn through line 19 contains the8320 llbs/hr. H2SO4 originally present in the alcohol black acid feed,as well as ftheY 2800 lbs/hr. H2S04 -formed =by oxidation of S02 t O S03in tower 10. As previously indicated, this acid contains 6.1 wt. percentN203 in solution and enters line 19V at :a temperature of 160 F., Theacid in line 19 is q u-ite clear but may still contain traces ofcarbonaceous matter, which impart a yellow or brown color to the naldenitrated acid, if not removed. In order to rernove the last traces of.carbonaceous matter before denitration, the `acid from line 19 ispassed through ahea-ter H-l, inV which the 'acid is heated from 160 to300 F. From H-1, the heated acid passes through line 20 into a holdingtank 3 7, in which the acid is held Vfor a period of Itime, usuallyabout 30 minutes, in order to complete the oxidation-ofthe last-tracesof carbonaceous matter. The completely decarbonizcd acid passes fromholding tank 37 through line v2 1 intoA the denitration tower 38.

1n order to obtain effective denitration of the decar- Yacidj-it isnecessary to dilute theacid ,-toabout "68%"H2SO4 strength 'and to stripthe diluted acid with of approximately 9 psfigjand'ata temperature ofapproximately 340 F. The gases romthe concentrator 39 enter thedenitration tower V38 through lin'e'27 under these conditions oftempenature and pressure, the following being the composition andquantity of these gases, which occupy a volume of 2720 c.f.m. at owngvconditions.

Gases entering dentragton tower 38 through line 2K7 a't 9 p.s..g. and340 F. f

Component Mol Mols] percent Hr.Y

6.8 29.1 1,864 7.8 '33.4 1,068 55.2 236.5 6,620 30.2 129.8 2,336Totai-.. 100.0 428.8 "n. ssfs The denitration ,tower 3.8 is operated insuch a manner that .the water vapor contained in the stripping gasesentering through line Z7 is completely condensed in .the lower sectionoltower 38. AThe amount o f.water rmoved from the acid inthe`concentrator' 39 is covtr'olled so that thedecarbonized 85%Y H2804entering Y tower as through une '21 is diluted to 68% H2so4, by

. condensationof thefwater passing lfrom concentrator 39 Y to-tower 38throughr line ZI-. The'denitrated 68% H2504 passesfrom the bottom oftower 38 into line Z5 at atemperature oliA 160 r Part of the 68% Vacidis recycled back to the -tower 3 8 throughlineV 24, cooler C73and.line.23 .and `the.denitrated"68% H2S04 product is transferred to theacid concentrator 39 through line 26.

YThe .reciitulad y68% aiff 'conledrcmnlf I0 E. in cooler Q -3 andthecooledacid,-amounting t0 @mompelt/,144300 lbs/ hr2 isineted intov tower384 at ,apoint between `the upper and lower packed sec.- tions. Therecirculated 68% H2S04 is mixed at this point with -the 13,880 lbs/hr.ofdecarbonized V85% H2804 containing `6.1% N202, which enters tower 38through line 2 1 at a temperature of 300? F., at a point located abovethe upper packed section of tower 38. As the hot 85% H2S04 flows downkoverthe packing in the upper sectionof tower 38, it scrubsthe gasespassing upwards from the lower section and removes the small amountY ofWater vapor remaining in the gases leaving the lower section of tower38. The blending of the 85% 'H2504 from line 21 .with the 68% H2504 fromline v2 3, Vresults inv the formation of acid of approximately69 l.tionv of tower 38. As the diluted acid meets the gases entering tower 38through line 27, the water vapor contained in the gases is condensed,with the result that the acid is further diluted from 69% H2804 toapproximately 68% H2804. The diluted acid ows over the packing in thelower section of tower 38 at a temperature of approximately 160 F., atwhich temperature the 68% H2804 can be stripped very completely ofnitrogen oxides by means of the noncondensible gases entering tower `38through line 27.

The temperature of the denitrated 68% acid leaving Vthe bottom of tower38 is controlled by the amount of 68% acid recirculated to tower 38through line 24, cooler C3 and line 23. Cooler C3 must remove all theheat liberated in tower 38 by the following processes:

(1) The cooling of the 85% H2804 from 300 F. to 160 P. and the heat ofdilution of 85% H2804 to 68% H2804.

(2) The cooling of the water vapor in the gases enter- Aing tower 38through line 27 and the latent heat Vof condensation of the water vaporat 160' F. Y

(3) The cooling of the noncondensible gases entering vtower 38 throughline 27 from 340 to 300 F.

Wt. Lbs/hr. percent H2801 67. 8 l1, 120 H20 32. 2 5, 292

Total 100. 16, 412

The hot gases entering concentrator 39 through line 29 are formed by thecombustion ofV sulfur with air in combustion chamber 36. Approximately932 lbs/hr. sulfur, charged to combustion chamber 36 through line .35,are burned with 8800 lbs./hr. of air, which enters chamber 36 throughline 34. The combustion is eiected Vat a temperature of approximately1800 F. and under a pressure of about 10 p.s.i.g. The quantities andcompo- .sition of materials entering and leaving the combustion chamber36 are approximately as follows:

Feed Products Mol Mols/ Lbs./ Mol Mols] Lbs./ percent hr. hr. percenthr. hr.

The hot gases from combustion chamber 36 still contain 10.8 mol percentof unreacted oxygen, which is required to oxidize the S02 andcarbonaceous matter in tower and to provide about 5 mol percentunreacted t 10 oxygen in the vent gases leaving tower 10 through line12. The hot gases also contain 180 lbs/hr. water vapor, which waspresent in the air charged to chamber 36. This water vapor is absorbedin the acid in tower 38, together with the water vapor formed inconcentrator 39, by concentration of the denitrated acid from 68% H2804to 78% H2804. Approximately 2156 lbs/hr. water are vaporized from thedenitrated acid in the concentrator 39, so that the total. quantity ofwater entering tower 38 through line 27 is 2336 lbs./hr.

In addition to removing water from the denitrated 68%l H2804, theconcentration of the acid at 340 F. in concentrator 39, provides anadditional safety factor for the complete removal of the last traces ofnitrogen oxides from the decarbonized, denitrated acid product. The acidproduct flows from concentrator 39 through line 31to a pressureequalizing tank 40, in which a level of hot acid product is maintained.Tank 40 is connected to concentrator 39 by means of a pressureequalizing line 30, through which gas can ilow as required to equalizethe pressures in tank 40 and concentrator 3 9.

The nal acid product ows from tank 40 to storage through line 32, coolerC-4 and line 33. Ihe composition and quantity of Yacid product is asfollows:

0f the nal 78% H2804 product amounting to 11,120 lbs/hr. H2804 (100%),approximately 75% or 8320 lbs./hr. H2804 was contained in the alcoholblack acid feed and the balance of 2800 lbs/hr. H2804 was made fromsulfur charged to the combustion chamber 36..

The decarbonization of alcohol black acid is an ideal application of theprocess, since the spent acid enters at 78% strength and the naldecarbonized product leaves as acid of the same concentration. However,the same process may be used for decarbonization of weaker or strongeracids, as well as for decarbonization of acids which contain a greaterconcentration of carbonaceous matter than alcohol black acid. Thusalcohol sludge acid, which consists of 50% H2804 containing about 6.7wt. percent carbonaceous matter, may be decarbonized and concentrated to78% strength by means of the process described. In the case of alcoholsludge acid, a greater quantity of water is produced by oxidation of thecontained carbonaceous matter, than is the case with alcohol black acid,which contains only 0.5% carbonaceous matter in 78% H2804. It is alsonecessary to react a considerably greater proportion of the wateroriginally present in the 50% strength alcohol sludge acid with 803, toraise the concentration of the acid above 80% H2804 for decarbonizationin tower 10, than is the case with 78% strength alcohol black acid. Theoverall effect is to produce a considerably greater proportion of newacid Ifrom sulfur or S02 when decarbonizing alcohol sludge acid, thanwhen decarbonizing alcohol black acid.

The following table shows the composition of alcohol sludge acid, thequantity of water which is formed by oxidation of the carbonaceousmatter and the total amount 4of water which must 'reacted Wit l 1 SO3;to rai se tht? 'concentration of the iacid mixtureto'l HZS'O.YA

me overall @mentation to 85% H2504, a pwiu be neces- Sary to react atotal of 46.63 tous of wateijWith'SO to 85% HZSOgaS follows;

E'or only 0.34 to 1. The plant-would be operated in ex'- ctly the same'manner for alcohol sludge 4acides for ale cohol black acid, except thataV greater Yproportion Vof sulwould be burned Vper unit of H2804 .in thespent acid In cases where a relatively Weak acid, suclras alcohol sludgeacid, is to be decarbonizedyitris sometimes desirable to increase thestrength of the spent acid,"be'fore it s-charge'd to the decarbonizationunit, by blending it with Va strong spent acid, such as spent alkylationacid, which contains about 88 H2502, 4% and carbonacfeous matter. Thusif 1 ton of alcohol sludgcacid is blended with 2 tons of spentalkylation citL: Kthe blend of spent acids will contain 74.3% H2SO4'an'd7.4% carbonaceous matter, as indicated in the' followigtable'i A160101sludge spent' xikyiai siem Ania Acid tion A` d Blend Component 1'1"1 1Wc, Tous' Wt. Tous Wt.VA Tous Percent Percent Y Percent .41 ss 1.767423. 2.23 0 47 4 0. 0s is. a 0. 55 0.06 s u. 16 7.4 0.22 11500 i V100f'ioc 100.0 3.00

if' 1001515 of the abpv'spent gein bleniis; decarbbpized according tothe process described; the' requi' ments'. o'f S03V and thenew acidproduced will be Shown il the The limit vto the amount o spnt alkylationacid that lcan be added to a system of Lhistype; vor' hcatayailable forconcentrating denitrated -68%' H2804, to 78% H2804. The additionL 'ofspent alkylation A acid ,to the process,y can thus educgbut not entirelyeliminate, the useof Vsulfur for collbus'tionv to provide heat for acidconcentration. Sie l"forix'lation of new` acid, by oxidation of S05 toS03, isY always the most desirable ratio being about 1 ton of H28044from S02 to 3 tons of HSO4 from the spent acid feed, as in thedecarbonizaticin` of alcoholblack acid.

aforesaid, theV decarbonization process embodied herein `lis -iiormallycarriedout atetemperaturesY of from about 1.0010 about 300thoughtemperaturesbbfe 300 F. may be employed Howeven-it is prfeed todecarbonize spent acids at temperatures below 300? F., withapproximately 80 to HSOg about 5 to 7 percent N203'- solution; `sinceS05 and oxygen dissolve at the desired ate oxidation ofSQg andcarbonaceous matter proceed at ara d rate u'nde'r these conditions. Afuthrimportant con eratonfor operating at reduced temperatures is thefacttht problems with materials of construciofi are by olerting at lowertemperatures;

When present in the pf proportions, NQ andlNOz dissolve in Siflf'ric'edd Oetemlg' 801 f 991%; HiQr f form N203 in amounts suitable forcarrying out this invention. AlthcmghA it isnotiritended that theinvention Yfollowing tables bebound by anytheory relating to themechanism of the f Y s n N f reactions that occur to' effctth'edecabonization of spent 85,7 S`0 Y acids, it is believed Y`thatwthedissolved N203 reacts with o z 4 Y Y viraerdlro; i E?itt 11fv jltxgessWtear 55 SAO?, or H2S0`4 1n the system to f orm a stab le complexcomponent tlt oxiiioof Acid Binai ii, hav1f1g. e vap0rr fs$uflower thaefhet; 9 f lf 29;0ref Blend, Carbnceous Y A Y* Produce 85% thelndividualox1des;N0'orNO3; that oxidation of. the Tous MatterTons" wt.rr'onsY HSO"TDS SO2 to S031teduesllheC drpl'w a lowevstatf f 'percent'oxidation; and thaty the dissolvedl oxygen immediately re- 60 stores thecomplex to its vformer state of oxidation, .whereg I-. in *by*liberation of less soluble; lower oxidesf niggen 714 from rthev the'solution ds'no; occurY tcpw vany `inpor'tfmt extent; andv that thecomplex of N505 with 803-1' or H2804 mal 10M 9'5 10 'su 14'? serves alsoto oxidize-.th'ecarbnacecus matter in7 the y 6'5` spent acid. Excess S03Re fvelgiggll' lgrls- In practice of an embodiment of the inyention asafore- Water', mired, described, in which formation of sulfuric acld andde- TOIS TQlS Wt T carbonization of spent sulfuric Aacid occursimultaneously Y i gter' ons and in which SO2-containing feed gases areemployed for 70 concentrating and denitrating the decarbonized acid,85.. 40,8 numerous advantages result. Thus by Vpractice of the 38-j3'jf1 51 inyention in an embodiment as aforedescribed, external r Y wheat0.1i fuel requirements for concentrating the denitrated 109; '4&0""'a'cidfom 68% to 78% sulfuric acid are eliminated or 'ser' .75,gillyminimized, as the heat required for this purpose is provided bycombustion of sulfur to supply SO2 to the system.

Although the use of nitric acid has been described for supply of make-uprequirements of nitrogen oxides -to the system, other sources ofnitrogen oxides may be used. Thus oxides of nitrogen, produced bycatalytic oxidation of ammonia, may be introduced directly into tower10, instead of adding an aqueous solution of nitric acid.

With reference to the strength of acid used for the decarbonizationtreatment, it is preferred Athat the acid have a sulfuric acid contentof about 83 to 85%, although an acid of 80 to 90% strength may be used.An acid of not over about 85% H2804 strength is preferred, however, aswith more concentrated acids, the solubility of SO2 in the reactionliquid appears to decrease, resulting in increased losses of SO2; andwith acids of less than 83% concentration of H2804 and less than about 5weight percent of dissolved N203, the partial pressures of nitrogenoxides above the acid solution appear to increase, with correspondinglyincreased losses of nitrogen oxides.

While there are above disclosed but a limited number of embodiments ofthe invention herein presented, it is possible to produce still otherembodiments without departing from the inventive concept hereindisclosed, and it is desired therefore that only such limitations beirnposed on the appended claims as are st-ated therein.

What is claimed is:

1. A process for decarbonizing a spent sulfuric acid containingcarbonaceous matter and about 80 to 90% sulfuric acid which comprisescontacting said acid, containing from about 3 to about 10 weight percentof dissolved dinitrogen trioxide, at a temperature of from about 100 toabout 300 F. with air in amount to maintain an excess of free oxygen andsulfur dioxide to dissolve sutiicient oxygen and sulfur dioxide in saidacid solution whereby carbonaceous matter in said solution is oxidizedto carbon dioxide and water and simultaneously dissolved sulfur dioxideis oxidized to react with water in the solution and form sulfuric acidin an amount sufhcient to maintain the sulfuric acid content of saidsolution at from about 80 to about 90 weight percent, and recovering adecarbonized sulfuric acid of about 80 to 90 weight percent of sulfuricacid containing dissolved dinitrogen trioxide.

2. A process, as defined in claim 1, wherein the decarbonized sulfuricacid is denitrated to remove dissolved oxides of nitrogen.

3. A process, as defined in claim 1, wherein the acid solution subjectedto contact with air and sulfur dioxide contains from about 5 to about 7weight percent of dinitrogen trioxide in solution, and the acid solutionis maintained at a sulfuric acid concentration of from about 82 to about86 weight percent.

4. A continuous process for decarbonizing spent sulfuric acid containingcarbonaceous matter which comprises continuously adding said spent acidto a solution of sulfuric acid of from about 80 to about 90 weightpercent of sulfuric acid and containing from about 3 to about l0 weightpercent of a dissolved nitrogen oxide,

.continuously contacting the resulting mixture at from about 100 toabout 300 F. with an excess of a free oxygen-containing gas, sulfurdioxide and an oxide of nitrogen to dissolve oxygen, sulfur dioxide andnitrogen oxide in said mixture, whereby carbonaceous matter is oxidizedto carbon dioxide and water and simultaneously dissolved sulfur dioxideis oxidized to sulfur trioxide for reaction with water in said mixtureto form sulfuric acid in an amount suicient to maintain the mixture at asulfuric acid content of about to about 90 weight percent, andwithdrawing from said mixture a substantially decarbonized sulfuric acidof from about 80 to about 90 weight percent of sulfuric acid andcontaining a dissolved oxide of nitrogen, said withdrawal ofdecarbonized acid being correlated with the addition of spent acid tothe sulfuric acid containing the dissolved oxide of nitrogen such thatthe resulting mixture contacted by the aforesaid gases is maintained ata sulfuric acid content of from about 80 to about 90 weight percent andcontains from about 3 to -about 10 weight percent of an oxide ofnitrogen.

5. A continuous process for decarbonizing spent sulfuric acid containingcarbonaceous matter which comprises maintaining at from to 300 F., inthe bottom portion of a vertically elongated oxidizing zone, a sulfuricacid solution of from about 80 to 90 weight percent of sulfuric acid andfrom about 3 to about 10 weight percent of dissolved dinitrogentrioxide, continuously adding spent acid containing carbonaceous matterto the top portion of said zone whereby said spent acid ows downwardlyand admixes with said sulfuric acid solution, continuously contactingsaid solution in the bottom portion of said oxidizing zone with anexcess of air, sulfur dioxide and an oxide of nitrogen tocountercurrently contact the downwardly flowing acid admixture anddissolve oxygen, sulfur dioxide and nitrogen oxide in said admixturewhereby dissolved oxygen oxidizes carbonaceous matter in the admixtureto carbon dioxide and water and simultaneously dissolved sulfur dioxideis oxidized to sulfur trioxide which reacts with water in the admixtureto form sulfuric acid in au amount suiiicient to maintain the acidsolution in the bottom portion of said zone at a sulfuric acid contentof from about 80 to about 90 weight percent and to maintain from about 3to about l0 weight percent of a dissolved oxide'of nitrogen in said acidsolution, and continuously withdrawing from the bottom portion of saidzone a substantially decarbonized solution of sulfuric acid of about 80to about 90 weight percent sulfuric acid in which from about 3 to about10 weight percent of dinitrogen trioxide is dissolved.

6. A process, as defined in claim 5, wherein the decarbonized sulfuricacid containing dissolved dinitrogen trioxide is denitrated bycontacting the acid with a gaseous mixture comprising sulfur dioxide andoxygen, to provide a denitrated sulfuric acid and a gaseous mixturecomprising oxygen, sulfur dioxide and oxides of nitrogen.

7. A process, as defined in claim 6, wherein the gaseous mixturecomprising oxides of nitrogen, sulfur dioxide and oxygen is utilized asthe gaseous mixture for contacting the acid solution in the bottomportion of the oxidizing zone.

8. A process, as defined in claim 6, wherein the gaseous mixtureutilized for denitrating the decarbonized acid is prepared by combustionof sulfur with excess air.

References Cited in the le of this patent UNITED STATES PATENTS2,390,316 Mottern Dec. 4, 1945 2,588,331 Titlestad Mar. 4, 1952 FOREIGNPATENTS 348,866 Great Britain May 21, 1931 672,723 Great Britain May 28,1952

1. A PROCESS FOR DECARBONIZING A SPENT SULFURIC ACID CONTAININGCARBONACEOUS MATTER AND ABOUT 80 TO 90% SULFURIC ACID WHICH COMPRISESCONTACTING SAID ACID, CONTAINING FROM ABOUT 3 TO ABOUT 10 WEIGHT PERCENTOF DISSOLVED DINITROGEN TRIOXIDE, AT A TEMPERATURE OF FROM ABOUT 100 TOABOUT 300*F. WITH AIR IN AMOUNT OT MAINTAIN AN EXCESS OF FREE OXYGEN ANDSULFUR DIOXIDE TO DISSOLVE SUFFICIENT OXYGEN AND SULFUR DIOXIDE IN SAIDACID SOLUTION WHEREBY CARBONACEOUS MATTER IN SAID SOLUTION IS OXIDIZEDTO CARBON DIOXIDE AND WATER AND SIMULTANEOUSLY DISSOLVED SULFUR DIOXIDEIS OXIDIZED TO REACT WITH WATER IN THE SOLUTION AND FORMM SULFURIC ACIDIN AN AMOUNT SUFFICIENT TO